The aim of this project is to study controlling factors in the muscle excitation-contraction process and to understand the evolution of and recovery from muscle fatigue. Fatigue is produced in single muscle fibers by tetanic stimulation. We are interested in relating the duration of stimulation and tension output to the rates of consumption of available substrates like glycogen, ATP, and phosphocreatine. Furthermore, tension output and kinetics of recovery will be related in turn to the instantaneous metabolic profile of the fibers. Previous work on metabolic control of contraction has been limited because it has been done on whole muscles, with nonhomogeneous fiber populations.